This invention relates to lighting systems generally, and more specifically, to an improved perimeter lighting system for producing more effective room lighting. A perimeter lighting system involves light fixtures located near the room walls, and may be employed primarily for wall lighting or for general room illumination, by reflection of the light from the walls. The importance of the invention is seen when the need for accurately determining where and how much lighting is necessary for effective illumination at a minimum of overall lighting expense is considered.
Maximizing lighting effectiveness is thus important because of the desirability of minimizing the amount of lighting used, that is, raw lumens, and hence, minimizing lighting expense, initially as well as in the long run. For example, a system which is twice as effective as an otherwise comparable system may require only half as many footcandles to provide the same amount of effective light, that is, to enable persons in the room to see and perform the same tasks as effectively. The benefits are (1) cutting the power requirements for the lighting system up to one-half, (2) eliminating some of the heat that must otherwise be removed from the room, (3) rendering easier the control of the luminance in the room, since the fixtures need produce less light, and (4) attaining better visual comfort. Further, utilization of more effective lighting systems results in benefits to society as a whole, including reductions in power requirements, thermal pollution, and brown-out and black-out hazards, as well as a lower consumption of natural resources.
In order to evaluate the effectiveness of various lighting systems, comparative standards are needed. The Illuminating Engineering society (IES) has established a system that compares the performance of any lighting system to the Equivalent Sphere Illuminaton (ESI) that would be attained if the task were illuminated by a perfectly and uniformly luminous system, such as would be attained inside a photometric sphere. IES standards are generally based on ESI levels. The ESI system applies specifically to the "standard school task" of reading pencil handwritting on tablet paper, but more generally to tasks with similar characteristics, such as typing and drafting, and reading printed matter and duplicated material. Lighting systems which give superior performance for the standard school task will also perform well for these other tasks.
ESI is actually the effective lighting level of a system; it is the end product in terms of performance of a given lighting system. That is, it is the amount of light from a perfectly uniform systems that gives the same visual performance as the system being designed. For example, if a system is producing a "raw footcandle" level of 200, with an ESI of 84, this means that 84 footcandles from a perfectly uniform system would provide the same visual effectiveness as is provided from the 200 footcandles of the system being studied. The effectiveness of the system (42% in this example) is the Lighting Effectiveness Factor(LEF). This factor is most important in determining which lighting system is best, but is dependent on a more basic factor -- the Contrast Rendition Factor(CRF). The CRF is not dependent upon the raw footcandle illumination level, and thus gives a direct means for comparing the relative effective values of different lighting systems. The CRF is dependent only upon the quality of the lighting systems, that is, on the geometry and room conditions, and thus is an expression of the relationship between the contrast of the standard school task under a given test lighting system and of the standard school task under the perfectly uniform system (ESI). Certain lighting systems will achieve CRFs in excess of 1.0.
Computation of CRF is very difficult, to the point of requiring a computer for accurate, practical determinations. However, it is not really necessary to compute CRF to select designs with high visual performance, as a knowledge of how CRF behaves with different geometries and types of lighting equipment is sufficient for selection of the most promising combinations, which can then be evaluated. The lighting system geometry has by far the greatest effect of all the factors on the visual performance of a lighting system.
The different geometries which must be considered are the different degrees of symmetrical luminaires, that is, the symmetrical variations of the output of the various complete light units. There are basically three degrees of symmetry relevant to luminaires: axially symmetrical, bisymmetrical, and monosymmetrical, illustrated in FIGS. 1, 2, and 3, respectively.
A luminaire which has the same candlepower distribution in every vertical plane through the center of axis of the luminaire is said to be axially symmetrical. That is, every vertical plane through the center of an axially symmetrical luminaire will have an identical candlepower distribution pattern, as shown in FIG. 1. Although the term asymmetrical is sometimes used to denote any degree of symmetry that is not absolute, that is, not axially symmetrical, the more precise designations of these degrees are bisymmetry and monosymmetry. General asymmetry is a complete lack of any symmetrical pattern.
However, any vertical plane through the axis of a bisymmetrical luminaire has the same candlepower distribution on each side of the axis, but each vertical plane (through the axis, but at different rotational angles) has a different distribution curve. That is, the distribution pattern is symmetrical through each and every vertical plane, but the candlepower distributions vary depending upon the angular attitude of the vertical plane, as shown in FIG. 2.
Likewise, a monosymmetrical luminaire has identical candlepower distribution patterns on either side of the horizontal axis through the direction of primary light distribution. That is, the light pattern is symmetrical on (1) the one vertical plane through the vertical axis at a perpendicular angle to the effective direction of primary light distribution, and (2) infinite vertical planes at various distances from the vertical axis, and perpendicular to the horizontal axis in the direction of primary light distribution, as shown in FIG. 3.
The primary light distribution patterns for both the axially symmetrical and bisymmetrical luminaires are ordinarily directed vertically downward, as shown by the fixture 1 in FIG. 4. The primary light distribution pattern for the monosymmetrical luminaire is generally directed outwardly from the center, in the same direction as the axis about which the distribution pattern (as seen in the imaginary vertical planes referred to above) is symmetrical, as shown by the fixture 2 in FIG. 4. The symmetrical pattern of the light distribution of fixture 2 in FIG. 4 would be seen on the vertical plane extending perpendicularly through the FIG. 4 page, intersecting the axis of symmetry of the light distribution pattern.
When either axially symmetrical or bisymmetrical luminaires are utilized, more light is directed downward, directly toward the work task area, than toward the wall for reflection toward the work task area at a shallow angle (to the horizontal). In contrast, a monosymmetrical luminaire aimed at the wall results in substantially more light being reflected off the wall toward the work task area. The wall-reflected illumination is more effective in producing the ESI than is the light from overhead. Hence, the monosymmetrical light distribution pattern, if properly directed, will result in the greatest ESI and CRF values.
Previous lighting systems have generally employed axially symmetrical reflecting fixtures. The effectiveness of their application in areas where wall reflection would otherwise provide substantial improvements in room lighting is limited by their axially symmetrical light distribution patterns. Various arrangements have been attempted with axially symmetrical fixtures in an effort to achieve CRFs in excess of 1.0. Although CRF is a ratio of a given system under study to a perfectly uniform system, it is not altogether uncommon to produce a CRF value greater than 1.0, since certain arrangements involving light reflective walls can result in greater LEFs than would the uniform system. For example, a room lighted by a direct-indirect lighting system with typical walls that result in fairly diffuse reflection, as shown in FIG. 10, may result in CRFs in excess of 1.0 over most of the room.
Direct-indirect lights typically provide 40-60% of their output downward, and 40-60% upward to be reflected off the ceiling toward the working area of the room, and for peripheral fixtures, toward the wall and then toward the works tasks. Suspended direct-indirect luminaires thus provide substantially equivalent amounts of their output upwardly and downwardly, with very little light emitted at angles near the horizontal. No light is shown in FIG. 10 as being directed downwardly because the comparative interest in that illustration is in the light reflected off the ceiling and wall.
Suspended semi-indirect luminaires are depending fixtures aimed back toward the ceiling, with 60-90% of their output directed upward for reflection off the ceiling and walls, and 10-40% directed downward to produce a luminaire luminance that closely matches that of the ceiling. These fixtures will generally exceed the performance of direct-indirect lighting systems, since less light is reflected or irradiated from directly overhead, as more light is reflected off the walls, at least for the perimeter area fixtures. Pure indirect lighting results in 90-100% of the output being directed toward the ceiling and upper side walls, and 0-10% being directed downward.
A system that eliminates the loss in the reflection off the ceiling can result in even greater CRF values, for the light from the ceiling to the task is less effective than light reflected directly off the wall. To achieve the highest CRF values under most conditions, the light must illuminate the work task area in a near horizontal direction, that is, from a reflecting surface at an elevation only slightly above the work task plane. This requirement that the light source (reflective wall) be slightly above the work plane effectively precludes use of ceiling mounted direct lighting, which results in 90-100% of the light output being directed downward, the concentration depending on the reflector material, finish, and shape. Luminous ceilings involve direct lighting, but are poor for providing adequate illumination from the near-horizontal zone (with respect to the work task area). That is, the CRF (at the location of the work task) produced by the ideally effective ESI level may be exceeded still further (with no additional light source) by an arrangement involving quasi-indirect lighting and light-reflective walls. Quasi-indirect lighting thus eliminates the reflection off the ceiling since the fixtures are recessed in the ceiling, and are aimed directly at the walls, at such an angle that the maximum amount of diffuse light is attainable from reflections off the wall at an elevational range (on the vertical wall) that provides the greatest CRF values in the work task zone. The prior art includes recessed symmetrical fixtures, but their distribution patterns do not provide as much light to the reflective walls as do equivalent monosymmetrical fixtures.
The LEF of the lighting used for any particular task is determined only partly by the amount of light which is present. More important than the amount of light is the adaptation of the light to the particular task being performed. If reflection from a particular task renders it difficult to see well, and if it is feasible to move the task with respect to the light, then the solution for increasing the LEF is obvious: move the work task or the light source. However, if the light source is immovable, or if it is necessary for the viewer to see in several different directions, it may well be unfeasible to move the work task and/or light source. In such a case, more effective lighting is necessary.
In order to provide effective lighting for work tasks in a given area, the light must come from directions other than directly above the work task. The most desirable location of illuminating sources is near the horizontal work place, that is, above the working area, at an angle in a range from 0.degree. to 30.degree. to the work place. In order to provide such illumination by means of ceiling mounted fixtures, the light must be reflected off the walls as well as come directly from the fixtures (in the more central areas of the room). Moreover, the elevation on the wall from which the main light distribution is reflected is of paramount importance in achieving a high ESI value.
Because it is desirable for the illumination to bounce off the wall rather than the ceiling, spot-type lights may be used more effectively if they are aimed directly at the wall (at the optimal angle) rather than indirectly via the ceiling. Directed spots have been used in room settings for decoratively featuring pictures or other artifacts on walls, but it has generally been thought unfeasible to use direct lighting as a useful method of generally illuminating a room for typical work tasks. Even the so called wall washes, by which an entire wall panel is lighted, produce relatively little useful light for general lighting purposes, that is, for typical work tasks. The problem is usually that the light fixtures are positioned so close to the wall that the light from the fixtures strikes the wall at a relatively low angle (with respect to the vertical wall). Such close positioning effectively washes a substantial portion of the wall with light, and without reflecting excessive light back into the work task area of the room. In fact, the stated purpose of such arrangements is frequently to minimize general room illumination, to prevent such light from detracting from the artistic high-lighting purpose for which the wall is illuminated.
Perimeter lighting with one or more direct-indirect light fixtures dropped from the ceiling such that 40-60% of the light will radiate back toward the ceiling and be reflected off it to the wall, and in turn be reflected from the wall into the work task area of the room, generally provides more effective lighting for work tasks than direct lighting. A schematic of direct-indirect perimeter lighting fixture is shown in FIG. 10. Various indirect lighting systems have long been used in buildings with high ceilings, as ample height is a prerequisite to such a system. The lower ceilings of today's architectural designs often make it impractical and/or old fashioned to have drop lights for perimeter lighting systems. However, luminous ceilings (a form of direct lighting) are often prohibitively expensive, generally difficult to maintain, and often impractical because of construction code requirements concerning the use of large areas of combustible plastics. In such cases, the only way to obtain high and uniform performance is by one form or another of indirect lighting. Quasi-indirect monosymmetrical perimeter lighting, that is, lights recessed in the ceiling along the periphery of the room, and directed downward toward the wall at a relatively high angle with respect to the wall, as shown in FIG. 11, so that the main light distribution pattern will be directed toward that area of the wall from which reflected light will produce the greatest CRF in the work task zone, can provide still further enhanced lighting effectiveness.
It is very desirable to have a low entry angle from the wall, that is, reentry of the light from the wall at a low angle to the horizontal work plane. Utilization of perimeter lighting with this low reentry angle allows attainment of the required ESI with fewer raw footcandles than with inferior lighting systems producing a lower CRF.
It is therefore a feature of the present invention to provide an improved perimeter lighting system using monosymmetrical light fixtures that may be mounted substantially flush with the ceiling, that is, recessed in the ceiling, with the lighting objective of illuminating the room, not merely the walls.
It is another feature of the present invention to provide an improved perimeter lighting system using monosymmetrical light fixtures to obtain a higher CRF than is achievable by means of a substantially identical peripheral lighting arrangement of axially symmetrical or bisymmetrical light fixtures.
It is still another feature of the present invention to provide an improved perimeter lighting system using monosymmetrical light fixtures for achieving optimum reflection from the walls of a room, for illuminating typical work tasks and for minimizing the effects of direct illumination which tends to cause veiling glare.